Such prior-art devices are based on the principle of optical light scattering and have a light source with high light stability, especially as an LED light source with long service life. The air carrying particles is sent via a sampling head through a sample tube, in which the sample is irradiated by the light at a finite angle relative to the direction of flow of the medium, usually at right angles hereto (i.e., at an angle not equal to 0°), such as preferably between 10° and 170°, preferably in the range of 80° to 100° relative to the direction of irradiation of the light.
The aerosol sensor is an optical aerosol spectrometer, which determines the particle size on the basis of scattered light analysis on the individual particle.
A defined gas volume flow is generated (in the range of 1.2 dm3/minute to 2.3 m3/hour) by means of a pump drawing in gas/air from the environment. The particles are moving individually through an optically defined measuring volume, which is illuminated homogeneously. A scattered light pulse, which is detected at the angle mentioned, is generated by every individual particle. The particle count is measured on the basis of the scattered light pulses. The particle concentration (e.g., particles per cm3) is calculated from the measured particle flow relative to the known volume flow (e.g., m3/sec). The level of the scattered light pulses is an indicator of the particle diameter.
Accordingly, each scattered light pulse is detected and assigned to a particle size based on its intensity, and the particle sizes of a plurality of channels, for example, 256 channels, are classified in a digital measuring system.
Such devices may have, furthermore, drying sections as well as sensors for detecting temperature, air pressure and relative humidity in order to thus rule out a distortion of measurement results due to condensation effects. Moisture compensation is carried out depending on the relative humidity of the air and the outside temperature.
Such a device for detecting the particle count concentration operates reliably and satisfactorily. However, the inner wall of the sample tube, through which the particle-containing gas to be analyzed flows, may become contaminated and cloudy due to the deposition of particles, so that the measuring means becomes decalibrated hereby. This means that smaller particles are actually counted in a measuring channel, which is intended for a certain particle size range, due to the attenuation of the scattered radiation that is caused by this.